Cochlear implant users report good system performance in quiet background conditions, but that performance degrades significantly with the addition of background noise. Similarly, most cochlear implant users have difficulty listening to music. As pitch is an important element of music, the perception of temporal fine structure in the audio signal is important for enjoying music.
The cochlear implant stimulation rate is recognized by cochlear implant users as pitch percept up to some given limit. For example, Baumann and Nobbe, Pulse Rate Discrimination With Deeply Inserted Electrode Arrays, Hearing Research 2004; 196 (1-2):49-57; incorporated herein by reference, describes that the ability to discriminate rate changes was limited to base rates up to about 283 pps. They also found no difference for basal and apical regions of the cochlea.
One cochlear implant stimulation strategy that transmits fine time structure information is the FSP strategy by MED-EL. Zero crossings of the band pass filtered time signals are tracked, and at each negative to positive zero crossing, a Channel Specific Sampling Sequence (CSSS) is started. Typically CSSS sequences are only applied on the first one or two most apical channels, covering the frequency range up to 200 or 330 Hz.
Another stimulation coding strategy that shows good performance in noisy conditions is known as Continuous Interleaved Sampler (CIS). For example, Kiefer J et al., Speech Understanding In Quiet And In Noise With The CIS Speech Coding Strategy (MED-EL Combi-40) Compared To The Multipeak And Spectral Peak Strategies (Nucleus), ORL J Otorhinolaryngol Relat Spec., 1996 May-June; 58(3):127-35; incorporated herein by reference described: “In the intersubject comparisons, the mean results in noise with the CIS strategy were superior to both the MPEAK and the SPEAK strategies.” But the CIS strategy is based solely on the envelope of the audio signal and phase information is disregarded so that fine time structure is not transmitted.
U.S. Pat. No. 7,039,466, incorporated herein by reference, describes a method for reducing the stimulation rate on basal channels: “[A]pically-located regions within the cochlea are stimulated at a reduced rate in order to conserve power.” This stimulation strategy attempts to reduce power consumption and does not make use of phase information from the audio signal. In addition, the order of rate-reduction is based on the refractory states of neurons and not on the pitch rate limit of a CI user.
United States Patent Application US 2007/0239227, incorporated herein by reference, describes a similar method for reducing stimulation rate, referred to as Frequency Modulated Stimulation (FMS). The FMS strategy does transmits information at integer multiples of zero-crossings or equivalent frequency counting, but does not restrict the stimulation rate according to a psychophysically measured pitch rate limit.